According to the theory of signal transmission, the demodulation of a signal corresponds to the mixing of the modulated signal with the sampling signal, and then the integration of the resulting product. The corresponding mixing step may be done using mixer circuits.
The basic element for many mixer circuits is a dual-gate transistor. An example of such a mixing circuit is presented by Sullivan et al. in “Doubly Balanced Dual(g)ate CMOS Mixer”, IEEE Journ. Of Solid-State Circ., Vol. 34, No. 6, 1999. The combination of at least four dual-gate transistors leads to a double balanced mixer for very high frequencies in the GHz range. Another example of an electronic mixer circuit based on dual-gate transistors is presented in U.S. Pat. No. 4,603,436 A (Butler, “Microwave Double Balanced Mixer”, 1986).
The above-mentioned mixer circuits have the disadvantage that several dual-gate transistors are required with additional load resistors, low-pass filters or differential amplifiers, which leads to a relatively complex circuit. Moreover, the size of the circuits does not allow the implementation as elements in a high-density array, such as, e.g., in the pixel structures of image sensors. Furthermore, for the intermediate frequency (IF) signal, an integration process of the IF signal would be required after the mixing process. As the number of phases would define the number of additional mixing elements, the required chip area increases. Furthermore, the mixing circuit is not useful for the mixing of very small modulated currents such as, e.g., photo-currents, because the mixing process is performed in the current domain instead of the less noisy charge domain.
In the publication EP-0'837'556 A1 (Wang, “Four terminal RF mixer device”, 1997) a so-called four-terminal-radio frequency (RF) mixer is described based on a MOS transistor having drain, source, gate and back-gate contacts. The radio-frequency (RF) signal and the local-oscillator (LO) signal are applied to the gate and back-gate contacts, respectively. A current flowing from the source to the drain of the transistor corresponds to the mixing result delivering the intermediate frequency (IF) signal. The disclosed mixing process has been reduced to only one transistor element and an additional load circuit. Different embodiments allow for partial and double balanced/unbalanced mixing, respectively. Although the basic mixing element has already been reduced to very compact size, the double balanced mixing still needs at least four discrete transistors of which each requires a gate, a source diffusion, a drain diffusion and an additional well implant. An in-phase/quadrature (I/Q) mixing circuit would look similar with only the signals being slightly different. The complex circuit consisting of many mixing transistors implies relatively large chip sizes to be used. Additionally, the four transistors have to match with high accuracy for the demodulation of very small input signals. Hence, the requirements put on the fabrication process are demanding and may reduce the expected fabrication yield dramatically.
In the field of demodulation devices, photo-sensitive sensors are known which use a device enabling the demodulation of the light waves. Such devices are described in the publications DE-44'40'613 C1, GB-2'389'960 A (Seitz, “Four-tap demodulation pixel”) and in the European patent application No. 04'405'489 (Büttgen et al., “Large-area pixel for use in an image sensor”). All these devices are directed to photo-currents with low intensities.